Most industrial complexes utilize piping and containment vessels to transport and deliver fuel, water, and other necessary solid and fluidic chemical materials. In installations, such as oil and gas fields, power stations, petrochemical plants, etc., the fluids and the environments in which they reside can be quite hostile. High temperatures and pressures exist in the presence of volatile, toxic, and corrosive chemical mixtures. As these substances are transported throughout a plant, they can cause both mechanical as well as chemical degradation of the piping and vessel infrastructure. The damage is seen in terms of physical material loss due to corrosion and erosion or a weakening of the infrastructure due to increased stress. Processes which cause material loss, degrade the structural and mechanical integrity of plants. In order to ensure safe and reliable operating conditions, such plants and facilities must be continuously inspected and monitored.
The key to maintaining reliability of an operating industrial complex is to develop and implement a regularly scheduled maintenance program. Minimal requirements that define basic safe operating procedures for maintenance and inspection are legislated in most areas of the world. However, these standards are not aimed at maintaining peak operating efficiency of the plant, nor are they specified to ensure maximal up-time for plant operations. Plant efficiency and up-time are important considerations in establishing a maintenance plan as there is a direct link to commercial profitability. Optimization of a maintenance plan requires the acquisition and detailed analysis of a multitude of both qualitative and quantitative data. The analysis is done to facilitate an assessment of the condition of the plant infrastructure and determine the fitness for service of specific components, vessels, pipes, etc. Once an evaluation is made, necessary adjustments can be made to the operating conditions of the plant and/or corrosion prevention program.
The cost and quality of the decisions is primarily driven by the precision, accuracy, and completeness of the measurements in the inspection. The better the precision and accuracy of the measurement, the better one can assess the condition, and predict if and when a failure will occur. The more complete the inspection information is, the more certain the conclusions will be since less reliance must be made on extrapolation and estimation. However, a fully condition-based maintenance program is difficult to deploy owing to the lack of availability of a complete quantitative survey of asset conditions. Furthermore many of the different inspection and monitoring modalities utilized in evaluating the fitness for service of a plant are unable to give a quantitative assessment of condition over large areas.
Digital imaging systems are becoming increasingly widespread for producing digital data that can be reconstructed into useful radiographic images. An exemplary system is described in our prior U.S. application Ser. No. 10/646,279, filed Aug. 22, 2003, now U.S. Pat. No. 6,925,145, the disclosure of which is hereby incorporated by reference herein. In addition, ultrasound (UT) technology gives very accurate measurements of material (i.e., wall) thickness. However, UT technology is in general limited to measurements over areas of point like dimensions. While this is sufficient for uniform general corrosion, localized corrosion and damage mechanisms that can produce such features as deep narrow pits and steep gradients of material erosion, are difficult to detect and measure accurately without the use of an imaging based modality. This is becoming less of an issue with the introduction of phased array ultrasound which allows UT data to be acquired over a more extended area. However, this fundamental technique is still a contact modality and is thus restricted to materials, and conditions, that support transmission of acoustic energy. UT measurements are sometimes difficult to deploy since a good acoustic coupling must be made between the transducer and the material under inspection. As such, any thermal insulation that is present on piping or vessels must be removed, or an inspection port need be installed so that direct access to the surface is available. Furthermore, piping that is insulated is quite often at an elevated temperature, and UT sensors may not be able to function efficiently under such conditions. Another problem with acoustic inspection methods is that multiple measurements must be made of the identical region over an extended period of time in order to track the evolution of these defects. These measurements have significant compromises in the ultimate precision that can be attained, and such measurements cannot be made in real-time in order to correlate with plant operating conditions. In many cases a non-contact modality, like radiography, is used to screen for regions where more detailed wall thickness information is required.
Other long term monitoring techniques, for example coupon evaluation, are known to provide information over a larger portion of a plant. However it is almost impossible to correlate coupons with specific plant operating conditions in real-time or to identify small regions of enhanced or accelerated corrosive activity. This technique is sensitive to conditions which are generally integrated over long time scales, and large areas. Specific information about small areas are not monitored well with this technique. Furthermore, unlike the other non-destructive testing (NDT) methods, installation and extraction of a coupon is an intrusive measurement.
Film based radiography systems suffer from a different problem. Although the raw data captured in a radiograph gives information over an extended area, it has been very difficult to extract quantitative information from the gray scale shading or rendering from a piece of film. Furthermore, the small dynamic range of film means that wide variations in material thickness cannot be imaged effectively in single exposures. The technique utilized in extracting a material thickness is to compare it with a calibrated shim or wedge of material of well-defined thickness. The general use of radiography has been to utilize its property of being a non-contract image based measurement to identify locations where features or defects exist. Since radiography is sensitive to the total path integral of the material between the source of radiation and the detector plate, the different contrasts and shading are used to extract qualitative information regarding such features and defects in the radiograph; that is, a lack of fusion in weld, or cracks show up as a variation in the gray shading. Quantification of this effect is very difficult. If a feature or defect is identified, the location is usually measured with an alternate modality, such as ultra-sound, to extract a true quantitative measurement.
While the quantitative behavior of the gray-scale is typically difficult to interpret, the dimensioning capability of measuring the spatial extent of a feature is quite easy because radiography is image based. As such, the size and dimension of a feature in the plane of the image can be easily deduced by comparing the feature to a reference object of known dimension. The precision of such a measurement is determined by the spatial resolution of the detector (i.e., film, imaging plate, detector, etc.), and the knowledge of the geometry of the source, object and film orientation. This is the fundamental principle behind the extraction of wall measurement thicknesses with the technique of profile or tangental radiography (radioscopy). In this technique, the wall thickness is surmised by taking a radiographic exposure tangental to the pipe, or vessel. A profile of the wall thickness is imaged and delimited by a contrast difference in the radiograph, which can be used to dimension the wall thickness. A simple correction is made for the shot magnification as defined by the relative distances between the X-ray source and detector, and the X-ray source and object under inspection. Correction by this factor allows the dimensioning of an object in absolute units to a high precision. Unfortunately this technique is restricted to extracting the wall thickness at the position perpendicular to the tangent of the pipe, and a complete series of shots need be taken to cover the complete area of the pipe, for example as described in U.S. Pat. No. 6,377,654.
The advent of digital technology has significantly increased the capabilities of radiography, although the fundamental technique of transmission radiography has not changed—that is, an X-ray radiation source illuminates an object under inspection. Typically, a radiation detector is placed behind the object so that it measures the X-ray spectrum transmitted through the object. The intensity of the transmitted spectrum is modulated by the material structure and density. The degree of intensity variation on the detector, or contrast variations, can be used to extract information about the material structure and integrity. With film radiography, the interpretation is generally qualitative. However, digital detectors provide a discrete numerical value that gives a measure of the transmitted X-ray flux on each individual sensing element, or pixel. This numerical value is proportional to the number of photons transmitted through the material under inspection and incident on the detector. The size and shape of the detector pixels, or sensing elements are a significant geometrical parameter, which along with the response of the detector components (i.e., material, electronics, etc.) determine the spatial resolution and sensitivity of the detector. This determines the minimum size of a feature that can be resolved. The discrete numerical value of the transmitted X-ray flux can then be mapped to a gray scale so that the image can be displayed in a manner similar to its film based renderings. As with film-based radiography, details of the structure of the object as well as dimensions of features can be determined, in similar manners; that is, by comparison to standard objects with known dimension. However, given access to the discrete numerical data of a digital image, procedures and algorithms can be automated to improve the speed, accuracy, and convenience of the measurements. These algorithms utilize well-known automated threshold detection and filtering algorithms to detect the spatial extent of features by quantifying the regions of contrast change within the radiograph.
Others have utilized digital radiography (DR) technology to compare a numerical gray scale value over a line or profile to a gray scale produced by a calibrated shim or wedge of known thickness. Such methods allow one to estimate the value of a material thickness. However the precision and accuracy of such a procedure is compromised in all but the most simple cases where the impulse (or thin material) approximation is valid. Many have taken the path of developing and patenting digital analogs of older methods, such as edge detection to facilitate automated dimensioning and tangental radiography, for example as disclosed in U.S. Pat. No. 6,377,654. However, limitations still exist in the application of radiography to extract precision absolute measurements on thicknesses over large areas.
Thus, there exists a strong need to develop a method for quantitatively transforming a complete radiographic image into an image representing the absolute thickness measurement, i.e., thickness map, of the material.